Method and a system for controlling service times of copied multicast cells in input modules of an asynchronous switching node

ABSTRACT

The invention relates to a method of controlling the service time of copied multicast cells in input modules of an asynchronous switching node. To minimize the time necessary to command the departure of all of N copied unicast cells, a queue (Q t ) is chosen in the input module as a reference queue in which the service time (SDT ref ) of one of the N copied unicast cells is controlled and in the N—1 other queues a service time is selected closest to the service time instant of the reference queue.

The invention relates to a method and a system for switching externaldata blocks received over transmission lines of an asynchronouscommunications node in a telecommunications system.

The expression “external data blocks” covers both packets of variablelength and cells of fixed length.

BACKGROUND OF THE INVENTION

A switching node considered in the context of the present descriptioncomprises input terminal modules and output terminal modules,interconnected by an asynchronous cell switching network. In theremainder of the present description the input and output terminalmodules are simply referred to as “input modules” and “output modules”,respectively.

In the switching node, an external data block received at an incomingport of an input module must be switched either to an outgoing port of agiven output module, in which case it is called a unicast data block, orto outgoing ports of a plurality of output modules, in which case it iscalled a multicast data block. External data blocks received in the formof variable length packets they are converted into internal format cellsin the input modules using techniques in the art. When they have beentransferred to the destination output modules via the switching network,the internal format cells are converted back into external data blocks,cells or packets. Within the switching node, the asynchronous switchingnetwork switches internal format cells, regardless of the external datablock type.

In the input modules of the node, depending on the data block type(unicast or multicast), there are two types of cells to be transferredacross the switching network: a first type of cell, called a unicastcell, which must be routed to a given single destination output module,and a second type of cell, called a multicast cell, which must be routedto N given destination output modules of the n2 output modules of thenode (where 1<N≦n2).

In the case of multicast cells, there are two prior art methods ofconverting a multicast cell into N cells to be delivered to the Nrespective destination output modules: in the first method, the inputmodule sends a single example of the multicast cell to the switchingnetwork, and it is the network which generates N copied cells from themulticast cell and routes each of the N copied cells to the N respectivedestination output modules required for the multicast cell. The secondmethod consists in generating N copies of a multicast cell in the inputmodule so that it then sends N unicast cells to the switching network,each cell being addressed to one of the N destination output modulesrequired for that multicast cell. The invention relates to this secondmethod in which the switching network transfers only unicast cells to agiven output module, as each multicast cell is converted into N unicastcells in the input module.

In the switching node, the streams of cells transferred across theswitching network can cause congestion in the network whenever theoverall bit rate of the cell traffic supplied to a given output moduleand coming from different input modules becomes excessive relative tothe bandwidth authorized for access to the output module.

The conventional way to prevent such congestion in the switching networkis for each input module to regulate the bit rates at which cells aretransferred to each output module of the node so that the overall bitrate of the cell traffic supplied to each output module is notexcessive.

To this end, each input module is provided with buffer memories fortemporarily storing cells, and the buffer memories are structured in theform of queues, each of which corresponds to a given output module.

That prior art technique is well suited to regulating multicast celltraffic bit rate when multicast cells are sent to only one outputmodule. Using the above technique to regulate the transfer across theswitching network of the traffic consisting of all of the cells, whichare all unicast cells, to their respective destination output modulescan therefore be envisaged a priori. To this end, in each input module,after a multicast cell is converted into N corresponding copied unicastcells addressed to N respective destination output modules of themulticast cell, each of the N copied unicast cells is then placed, alongwith the normal unicast cells, in the queue relating to its destinationoutput module. Accordingly, each stream of normal and copied unicastcells sent to a given output module from the corresponding queue of aninput module can be regulated by a predetermined cell service bit rateallocated to that queue.

However, the problem with implementing this prior art techniqueapplicable to unicast cells in each input module is to distributebeforehand the corresponding N copied cells for each multicast cell intothe required N queues.

For each multicast cell, the complexity of the process of conversioninto the N copied unicast cells to be placed in the respective queuesincreases with the number n2 of output modules of the node (since N≦n2).The process is therefore extremely complex for a high-capacity switchingnode having a large number of output modules. This leads to a circuitrysurface area that can be excessive in the case of a paralleldistribution or to a processing time that can be long in the case of asequential distribution. Moreover, a simple distribution of the abovetype cannot correlate in time the departure instants from the required Nqueues of the N copied unicast cells corresponding to a given multicastcell.

OBJECTS AND SUMMARY OF THE INVENTION

To eliminate this problem of distributing the N unicast cellscorresponding to each multicast cell, the invention relates to a methodand to a system for use in the input modules of the asynchronousswitching node to control the service times of normal and copied unicastcells in the queues relating to each of the streams of outgoing cellscorresponding to the subset of cells supplied by the switching networkto each given output module, which method also ensures that the overallbit rate of the traffic comprising all of the cells supplied to eachoutput module is not excessive.

The invention therefore concerns a method of controlling the servicetime of cells in input modules of an asynchronous switching node whichcomprises n1 input modules and n2 output modules interconnected by acell switching network that transfers each cell from an input module toan output module. A unicast cell is addressed to a single givendestination output module and a multicast cell is addressed to N givendestination output modules of the n2 output modules of that node, eachinput module itself converting each multicast cell into N correspondingcopied unicast cells respectively addressed to the N output modules.Each input module is provided with buffer memories for temporarilystoring cells and the service time of a normal or copied unicast cell ina buffer memory triggers its transfer from the input module to thedestination output module across said switching network. The servicetimes of all the unicast cells are commanded in queues in each inputmodule relating to each output module, the input module being controlledso that the respective service times of the cells sent from a givenqueue are separated by a time interval at least equal to the requiredservice time interval between two successive cells of the stream ofcells supplied by the switching network to the corresponding outputmodule.

This last condition guarantees that a predetermined cell service bitrate allocated to the corresponding queue is not exceeded for eachstream of cells supplied to a given output module.

In the preferred embodiment of the invention, in order to reduce thetime necessary for commanding the departure of all the N copied unicastcells (CUC_(a), . . . , CUC_(n)) from the N queues (Q_(a), . . . ,Q_(n)) relating to the N destination output modules of a multicast cell(MC), one of the N queues is chosen as a reference queue in which iscontrolled the service time, referred as the reference service time, ofone the N copied unicast cells corresponding to that multicast cell.Additionally, in each of the N−1 other queues concerned, the servicetime of each of the N−1 other copied unicast cells corresponding to thesame multicast cell is controlled by selecting a service time instant ofthe queue concerned which is closest to the instant of said referencetime controlled in said reference queue.

Accordingly, for each of the N−1 queues concerned, the temporal distancebetween the instant of said service time concerned and the instant ofsaid reference service time is at most equal to said service timeinterval of that queue concerned.

Then, in accordance with the invention, by using for each of the otherN−1 queues concerned a service time selected because it is that nearestthe instant of the reference service time, it is possible to distributeinto N queues the N copied unicast cells that correspond to the samemulticast cell and to command the respective departures of those N cellsin a reduced time interval. Additionally, the duration of the reducedtime interval can be determined as a function of the cell service bitrates allocated to the N queues.

The invention is particularly simple to put into practice and, firstly,minimizes the duration of traffic peaks generated in an input module bythe internal processing of each multicast cell and, secondly, provides amethod of determining the duration for which a multicast cell is storedin the buffer memory of the input module.

In one embodiment, the reference queue (Q_(ref)) selected is the queueallocated the lowest predetermined cell service bit rate from thevarious service bit rates allocated to the N queues relating to the Ndestination output modules of that multicast cell.

In this case, the selection of the reference queue and the selection ofsaid service times concerned in each of the N−1 queues concerned can bepredetermined from the instant of the service time that precedes thereference service time retained in the reference queue, and in thissituation, said service time concerned selected for each of the N−1queues concerned is the last service time instant of that queue whichprecedes the instant of the reference service time in said referencequeue.

In one embodiment, for each of the N−1 queues concerned, said servicetime concerned selected is the first service time instant of that queuewhich follows the instant of the reference service time in saidreference queue.

The invention also relates to an input module including means forimplementing the methods defined hereinabove.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features and advantages of the invention will become apparent fromthe following description of embodiments of the invention, which isgiven with reference to the accompanying drawings, in which:

FIG. 1 is a theoretical diagram of a switching node for implementing acontrol method according to the invention,

FIG. 2 is a diagram showing outgoing streams of cells supplied to agiven output module from different input modules in the switching nodeshown in FIG. 1,

FIG. 3 is a theoretical diagram of an input module of a switching nodeas shown in FIGS. 1 and 2,

FIG. 4 is a diagram illustrating an example of a sequence of successiveservice times of three queues in an input module involved in controllinga multicast cell addressed to three destination output modules relatingto those queues,

FIGS. 5 a and 5 b are diagrams illustrating the general operation of theFIG. 4 example of a method according to the invention,

FIGS. 6 a and 6 b are diagrams illustrating the operation of a firstembodiment of the FIG. 4 example of a method according to the invention,and

FIG. 7 is a diagram illustrating the operation of a second embodiment ofthe FIG. 4 example of a method according to the invention.

MORE DETAILED DESCRIPTION

The following abbreviations are used in the remainder of thedescription:

-   CUC: copied multicast cell,-   GOCR: granted output cell rate,-   ITM: input termination module,-   MC: multicast cell,-   OTM: output termination module,-   Q: queue,-   SDT: scheduled departure time,-   STI: service time interval,-   UC: unicast cell.    The following subscripts are generally used:    -   The subscript i designates any input module, e.g. ITM_(i),    -   The subscript j designates any output module, e.g. OTM_(j),    -   The subscript d designates a considered or given output module,        e.g. OTM_(d), and    -   The subscript c designates a concerned output module, e.g.        OTM_(c).

Firstly, the theory of an asynchronous switching node in atelecommunications system and the associated problems are brieflydescribed.

As shown in FIG. 1, an asynchronous switching node 1 in atelecommunications system includes n1 input modules ITM₁, . . . ,ITM_(i), . . . , ITM_(n1) which are conventionally connected to n2output modules OTM₁, . . . , ITM_(i), . . . , OTM_(n2) by anasynchronous switching network 2. This separate representation of n1input modules and n2 output modules can correspond either to physicallyseparate input and output modules or to logical separation of the inputand output functions in combined terminal modules providing both typesof function, in which case the numbers n1 and n2 of modules aregenerally equal.

The switching node transfers a data block received by any input moduleITM_(i) to any one or more output modules OTM_(j) via its switchingnetwork 2 in accordance with any internal routing mode, for example aconnection-oriented or connectionless routing mode.

An external data block received at an incoming port of an input modulemust be switched either to an outgoing port of a given output module, inwhich case it is called a unicast data block, or to outgoing ports of aplurality of output modules, in which case it is called a multicast datablock.

In the prior art, external data blocks received in the form of variablelength packets are converted into internal format cells in the inputmodules, typically by a process of packet segmentation. Once transferredto the destination output modules via the switching network, theinternal format cells are converted back into external data blocks,cells or packets, typically by a process of cell reassembly in the caseof external data blocks to be sent in the form of variable lengthpackets. Within the switching node, the asynchronous switching networkconsidered switches internal format cells, regardless of the externaldata block type, and each cell is transferred from an input module to atleast one destination output module.

Two types of cell therefore have to be considered in the input modulesof the node: a first type of cell, called a unicast cell UC, must berouted to a single given destination output module, and a second type ofcell, called a multicast cell MC, must be routed to N given destinationoutput modules from the n2 output modules of the node (1<N≦n2).

There are two prior art methods of converting a multicast cell into Ncells to be supplied to the N respective destination output modules: Inthe first method the input module sends the switching network a singleexample of the multicast cell MC, and the switching network is able togenerate N copied cells Uc_(a), . . . , Uc_(n) from a multicast cell andto route each of the N copied cells to the N respective destinationoutput modules required for that multicast cell. The second method,which is used here, generates N copies of a multicast cell in the inputmodule, so that the latter in fact sends to the switching network Nmulticast cell each of which is to be sent to one of the N destinationoutput modules required for that multicast cell.

FIG. 2 shows outgoing streams of cells supplied to a given output moduleOTM_(d) from input modules ITM₁, . . . , ITM_(n1) in the switching node1. There is a risk of the streams of cells transferred via the switchingnetwork creating congestion in the network whenever the overall bit rateof the traffic of cells supplied to an output module, such as the moduleOTM_(d), becomes excessive in relation the authorized bandwidth foraccess to the output module, given that the capacity to transfer cellsvia the switching network is physically limited by characteristicsspecific to the network.

A conventional way to avoid congestion in the switching network is foreach input module to regulate the bit rates at which cells aretransferred to each output module of the node so that the overall bitrate of the traffic of cells supplied to each output module is notexcessive.

To this end, using a technique known in the art, and as shown in FIG. 3,each input module is provided with buffer memories 3 for storing cellstemporarily and the buffer memories are structured in the form of aplurality of queues Q₁, . . . , Q_(i), . . . , Q_(n2) each of whichcorresponds to a given output module.

Using the above technique in an input module ITM_(i), after processingby input interface and internal cell conversion functions known in theart (and not shown in FIG. 3), the cells are distributed to the queuesQ₁, . . . , Q_(j), . . . , Q_(n2) as a function of their destinationoutput module OTM₁, . . . , OTM_(j), . . . , OTM_(2n). To extract a cellfrom a queue and send it to the switching network, a queue multiserver 4uses one of its service cycles to read the corresponding data in thequeue and extract it therefrom. A queue multiserver distributes itsservice cycles to the queues proportionately to the cell service bitrate GOCR_(j) allocated to each queue Q_(j) for transferring the datablocks to be sent to the corresponding output module OTM_(j). All thequeues of an input module are controlled by a queue manager unit 5assigned to that input module.

The composition of the output modules of the node 1 is not shown indetail here because each output module can be of conventional designwith a plurality of registers or memories to receive the data blocks,convert them into internal cells and then transmit those cells to theoutput interface functions of the input module.

The control functions required by the switching node 1 can be of anytype known in the art. The control functions are therefore not shown inthe figures, except for the queue manager unit 5 of an input module,shown in FIG. 3, which includes a portion of the characteristic meansused to implement the service time control method according to theinvention, here considered to constitute a dedicated control mechanismspecific to the input modules of the node 1. The queue manager unit 5 isprovided with appropriate hardware resources, including processors andmemories, and appropriate software resources, in particular, forimplementing the control method according to the invention.

The technique known in the art based on the use in the input modules ofqueues for each output module is suitable for regulating the traffic bitrate of unicast cells addressed only to a single output module. In aqueue Q_(d) relating to a given destination output module OTM_(d), theservice time SDT of a unicast cell UC, which triggers its transfer fromthe input module to the output module via the switching network, iscommanded at an instant determined as a function of the cell service bitrate GOCR_(d) allocated to that queue, which can be predetermined sothat all of the queues Q_(d) in the input modules ITM₁, . . . , ITM_(l),. . . , ITM_(n1) relating to the same output module OTM_(d) generate anoverall traffic bit rate of cells supplied to the output module that isnot excessive.

This technique can therefore be used to regulate the transfer of all thetraffic consisting of normal or copied unicast cells across theswitching network to the respective destination output modules. In eachinput module, after a multicast cell into N has been converted intocorresponding copied unicast cells addressed to N respective destinationoutput modules of the multicast cell, each of the N copied unicast cellsis placed, along with the normal unicast cells, in the queue relating toits destination output module. Accordingly, each stream consisting ofall the normal or copied unicast cells sent to a given output modulefrom the queue corresponding to an input module can be regulated by apredetermined cell service bit rate allocated to that queue.

However, the problem with implementing this prior art techniqueapplicable to unicast cells in each input module is to distributebeforehand the corresponding N copied cells for each multicast cell intothe required N queues, the complexity of the process of conversion intothe N copied unicast cells increasing with the number n2 of outputmodules of the node (since N≦n2), so that the process is extremelycomplex for a high-capacity switching node.

To solve this problem relating to the distribution of the N copiedunicast cells CUC_(a), . . . , CUC_(n) corresponding to each multicastcell MC, the method of the invention controls the service time of thenormal and copied unicast cells UC, CUC in the input modules ITM_(i) ofthe asynchronous node 1 in queues Q₁, . . . , Q_(j), . . . , Q_(n2)relating to each of the streams of cells supplied by the switchingnetwork 2 to each output module OTM₁, . . . , OTM_(j), . . . , OTM_(n2),minimizing the duration of the time interval needed to distribute the Ncopied unicast cells into the N respective queues Q_(a), . . . , Q_(n),and to command their respective departures.

The control method of the invention is described for an example of amulticast cell MC_(tuv) addressed to three (N=3) destination outputmodules OTM_(t), OTM_(u), OTM_(v). The FIG. 4 diagram illustrates anexample of a sequence of successive service times of each of the threequeues Q_(t), Q_(u) and Q_(v) in an input module relating to the threeoutput modules.

For each of the queues Q_(t), Q_(u) and Q_(v), the instants of theirrespective service times SDT_(t), SDT_(u), SDT_(v) are indicated in thefigure by black circles on three respective time lines on which timeflows from right to left. The position of their successive service timesis determined periodically by a respective service time intervalSTI_(t), STI_(u), STI_(v) between them. For example, for the queueQ_(t), the three successive service times SDT_(t/x), SDT_(t/x+1) andSDT_(t/x+2) represented in FIG. 4 are separated by the duration of theservice time interval STI_(t) required between the departures of twosuccessive unicast cells. The service time intervals STI_(t), STI_(u),STI_(V) of the queues Q_(t), Q_(u), Q_(v) are inversely proportional tothe respective cell service bit rates GOCR_(t), GOCR_(u), GOCR_(v),allocated to those queues. In the example shown in FIG. 4, the cellservice bit rates allocated to the three queues are such thatGOCR_(u)<GOCR_(v)<GOCR_(t), which leads to the situationSTI_(u)>STI_(v)>STI_(t) shown in FIG. 4.

The operating principle of the method according to the invention isdescribed with the aid of FIG. 5 a, which is based on the example ofcontrolling the multicast cell MC_(tuv) in the sequences of servicetimes from FIG. 4. The requirement to send this kind of multicast cellMC_(tuv) that arises at an instant in the central part of the diagram isconsidered. One of the three queues is then chosen as the referencequeue Q_(ref), for example the queue Q_(t) (so that in this exampleQ_(ref)=Q_(t)). The reference queue is used to send one of the threecopied unicast cells corresponding to the multicast cell MC_(tuv),namely the unicast cell CUC_(t) addressed to the output module OTM_(t)relating to the queue Q_(t), for example at the service timeSDT_(t/x+3), which thus constitutes the reference service time SDT_(ref)(so that in this example SDT_(ref)=SDT_(t/x+3)), the instant of which ismarked by a black square in the figure.

Additionally, to control, in each of the other two (N−1=2) queuesconcerned Q_(u) and Q_(v), the service time concerned of each of the twoother unicast cells CUC_(u) and CUC_(v) corresponding to the samemulticast cell, a service time is selected for the queues Q_(u) andQ_(v) that is closest to the instant of the reference service timeSDT_(ref)=SDT_(t/x+3) (marked by the vertical line) of the referencequeue Q_(ref)=Q_(t) such as the respective service times SDT_(u/y+1) andSDT_(v/z+2) in the figure, the instants of which are marked by whitesquares. Accordingly, for the two queues concerned Q_(u) and Q_(v), thetemporal distance between the instant of their service time SDT_(u/y+1),SDT_(v/z+2) selected in this way and the instant of the referenceservice time SDT_(ref)=SDT_(t/x+3) is at most equal to the service timeinterval STI_(u), STI_(v) of the queues concerned Q_(u), Q_(v).

The three copied unicast cells CUC_(t), CUC_(u), CUC_(v) that correspondto the same multicast cell MC_(tuv) are then distributed into the threequeues Q_(t), Q_(u), Q_(v) and the respective departures of the threecells are commanded in a reduced time interval. As indicated in FIG. 5a, the reduced time interval T_(a) during which the three copied unicastcells CUC_(t), CUC_(u), CUC_(v) are distributed and their respectivedepartures are commanded is delimited by the instants of the farthestaway service times, namely the service times SDT_(u/y+1) and SDT_(v/z+2)in the FIG. 5 a example.

FIG. 5 b, which is again based on the example of controlling themulticast cell MC_(tuv) in the FIG. 4 sequences of service times,illustrates an additional feature of the invention whereby the referencequeue Q_(ref) chosen is preferably the queue allocated the lowestpredetermined cell service bit rate GOCR_(min). In this case, it is thenthe queue Q_(u) that is selected as the reference queue Q_(ref) since,in this example, it has the longest service time interval STI_(u), andtherefore the lowest cell service bit rate GOCR_(min)=GOCR_(u). Thereference queue Q_(u) is used to send one of the three copied unicastcells corresponding to the multicast cell MC_(tuv), namely the unicastcell CUC_(u) addressed to the output module OTM_(u) relating to thequeue Q_(u), for example at the service time SDT_(u/y+1) which thenconstitutes the reference service time SDT_(ref), the instant of whichis marked by a black square.

To control the service time concerned of each of the other two copiedunicast cells CUC_(t) and CUC_(v) in each of the other two queuesconcerned Q_(t) and Q_(v), the service times closest to the instant ofthe reference service time SDT_(u/y+1) (marked by the vertical line) ofthe reference queue Q_(t) are then selected, such as the respectiveservice times SDT_(t/x+3) and SDT_(v/z+1) in FIG. 5 b. For these twoqueues concerned Q_(t) and Q_(v), the temporal distance between theinstant of their service times SDT_(t/x+3) and SDT_(v/z+1) selected inthis way and the instant of the reference service timeSDT_(ref)=SDT_(u/y+1) is at most equal to the service time intervalSTI_(t) and STI_(v) of the queues concerned. As indicated in FIG. 5 b,the reduced time interval T_(b) during which the three copied unicastcells CUC_(t), CUC_(u), CUC_(v) are distributed and their respectivedepartures are commanded is delimited by the instants of the farthestaway service times, namely the service times SDT_(t/x+3) and SDT_(v/z+1)in the FIG. 5 b example.

The advantage of this preferential selection of the reference queueQ_(ref) will become apparent in relation to an embodiment describedlater with reference to FIG. 7.

FIGS. 6 a and 6 b, which are also based on the example of controllingthe multicast cell MC_(tuv) in the FIG. 4 sequences of service times,illustrate an embodiment of the invention whereby the service timeconcerned SDT_(c) selected for each of the N−1 queues concerned Q_(c) isthe first service time instant of that queue which follows the instantof the reference service time SDT_(ref) in said reference queue Q_(ref).

FIG. 6 a illustrates an example analogous to that of FIG. 5 a insofar asconcerns the choice, from the three (N=3) queues, of the queue Q_(t) asthe reference queue Q_(ref) and of the service time SDT_(t/x+3) as thereference service time SDT_(ref). In this embodiment, the service timesSDT_(u/y+2) and SDT_(v/z+2) are then selected for the two queuesconcerned Q_(u) and Q_(v), as they are closest to and beyond the instantof the reference service time SDT_(t/x+3).

As indicated in FIG. 6 a, the reduced time interval T′_(a) during whichthe three copied unicast cells CUC_(t), CUC_(u), CUC_(v) are distributedand their respective departures are commanded is then delimited,firstly, by the instant of the reference service time SDT_(ref) and,secondly, by the instant of whichever of the service times concernedSDT_(c) is the farthest away from the service time SDT_(ref), namely theinstants of the service times SDT_(t/x+3) and SDT_(u/y+2) in thisexample.

FIG. 6 b illustrates another example analogous to that of FIG. 5 b withregard to the choice from the three (N=3) queues of a queue having thelowest cell service bit rate OGCR_(min), i.e. the queue Q_(u) in thisexample, as the reference queue Q_(ref), and of the service timeSDT_(u/y+1) as the reference service time SDT_(ref). In the sameembodiment, the service times SDT_(t/x+3) and SDT_(u/y+2) closest to andbeyond the instant of the reference service time SDT_(u/y+1) are thenselected for the two queues concerned Q_(t) and Q_(v).

As indicated in FIG. 6 b, the reduced time interval T′_(b) during whichthe three copied unicast cells CUC_(t), CUC_(u), CUC_(v) are distributedand their respective departures are commanded is then delimited,firstly, by the instant of the reference service time SDT_(ref) and,secondly, by the instant of the service times concerned SDT_(c) which isthe farthest away from SDT_(ref), namely by the instants of the servicetimes SDT_(u/y+1) and SDT_(v/z+2) in the FIG. 6 b example.

As a general rule, in this embodiment, choosing the service timesconcerned closest to and beyond the reference service instant guaranteesthat, irrespective of the relative position of the various sequences ofservice times in the N queues, the duration of the minimum time intervalT′ is most equal to whichever is the greatest of the service timeintervals STI_(c) of the N−1 queues concerned Q_(c), such as the servicetime interval STI_(u) or STI_(v) in the FIG. 6 a or 6 b example,respectively.

Furthermore, this embodiment has the advantage of being simpler to putinto practice, because distribution can be triggered at the referenceservice time SDT_(ref) by commanding the departure of the copied unicastcell CUC_(ref) corresponding to the reference queue Q_(ref) that hasbeen selected, the position of the service times concerned SDT_(c) beingpredetermined as those closest to the instant of the service timeSDT_(ref) starting from that instant.

FIG. 7, again based on the example of controlling the multicast cellMC_(tuv) in the FIG. 4 sequences of service times, illustrates anotherembodiment of the invention in which the reference queue Q_(ret) and theservice times concerned SDT_(c) in each of the N−1 queues concernedQ_(c) is selected in advance, at the instant of the service timeSDT_(ref′) that precedes the reference service time SDT_(ref) retainedin the reference queue Q_(ref) selected here as a queue having thelowest cell service bit rate GOCR_(min). The service time concernedSDT_(c) selected for each queue concerned Q_(c) is then the last servicetime instant of that queue which precedes the instant of the referenceservice time SDT_(ref) in said reference queue Q_(ref).

FIG. 7 illustrates an example analogous to that of FIGS. 5 b and 6 bwith regard to the choice, from the three (N=3) queues, of a queuehaving the lowest cell service bit rate GOCR_(min), which is the queueQ_(u) in this example, as the reference queue Q_(ref), and the servicetime SDT_(u/y+1) as the reference service time SDT_(ref). In thisembodiment, this reference queue Q_(ref)=Q_(u) is then selected inadvance during its service time SDT_(ref′)=SDT_(u/y), marked by a blacktriangle in FIG. 7, which precedes its reference service timeSDT_(ref′)=SDT_(u/y+1).

Furthermore, during this service time SDT_(ref′), the service timesSDT_(t/x+2) and SDT_(v/z+1) preselected for the two queues concernedQ_(t) and Q_(v) are those closest to and preceding the instant of thereference service time SDT_(ref)=SDT_(u/y+1). It may be noted that inthis embodiment, for each of the N−1 queues concerned, there alwaysexists at least one service time that can be selected during the timeinterval T″ between the service times SDT_(ref′), and SDT_(ref), sincethese two consecutive service times of the selected queue Q_(ref) arethemselves separated by a service time interval SDT_(ref) thatcorresponds to the lowest cell service bit rate GOCR_(min), and which istherefore at least as long as each of the service time intervals of theother N−1 queues concerned Q_(c).

In the example illustrated in FIG. 7, the reduced time interval T″during which the three copied unicast cells CUC_(t), CUC_(u), CUC_(v)are distributed and their respective departures are commanded is thendelimited by the instants of the service times SDT_(ref′)=SDT_(t/x+1)and SDT_(ref)=SDT_(t/x+2). As a general rule, in this embodiment,preselecting as the service times concerned SDT_(c) those closest to andpreceding the reference service instant SDT_(ref) also guarantees that,regardless of the relative position of the various service timesequences of the N queues, the duration of the minimum time interval T″is equal to the greatest of the service time intervals of the N queues,namely the service time interval STI_(ref) of the queue Q_(ref) selectedhere as having the lowest cell service bit rate GOCR_(min), such as theservice time interval STI″ in the FIG. 7 example.

This embodiment also has the advantage of being simpler to put intopractice, because it allows triggering of distribution at the servicetime SDT_(ref′) that precedes the reference service time SDT_(ref) andcommanding the departure of the N copied unicast cells during the timeinterval between the two consecutive service times SDT_(ref′) andSDT_(ref) of the reference queue Q_(ref), predetermining the position ofthe service times concerned SDT_(c) as those closest to and precedingthe instant of the service time SDT_(ref).

1. A method of controlling the service time of cells in input modules ofan asynchronous switching node which comprises n1 input modules (ITM₁, .. . , ITM_(i), . . . , ITM_(n1)) and n2 output modules (OTM₁, . . . ,OTM_(j), . . . , OTM_(n2)) interconnected by a cell switching networkthat transfers each cell from an input module to an output module, aunicast cell (UC) being addressed to a single given destination outputmodule (OTM_(d)) and a multicast cell (MC) being addressed to N givendestination output modules (OTM_(a), . . . , OTM_(n)) of the n2 outputmodules of that node, each input module itself converting each multicastcell into N corresponding copied unicast cells (CUC_(a), . . . ,CUC_(n)) respectively addressed to the N output modules (OTM_(a), . . ., OTM_(n)), each input module being provided with buffer memories fortemporarily storing cells and said service time of a normal or copiedunicast cell (UC; CUC) in a buffer memory triggering its transfer fromthe input module to the destination output module across said switchingnetwork, and the service times of all the unicast cells (UC; CUC) beingcommanded in queues (Q₁, . . . , Q_(j), . . . , Q_(n2)) in each inputmodule relating to each output module (OTM₁, . . . , OTM_(j), . . . ,OTM_(n2)), the input module being controlled so that the respectiveservice times of the cells sent from a given queue (Q_(j)) are separatedby a time interval at least equal to the required service time interval(STI_(j)) between two successive cells of the stream of cells suppliedby the switching network to the corresponding output module (OTM_(j)),in order not to exceed for each stream of cells supplied to a givenoutput module (OTM_(j)) a predetermined cell service bit rate (GOCR_(j))allocated to the corresponding queue (Q_(j)), characterized in that, inorder to reduce the time necessary for commanding the departure of allthe N copied unicast cells (CUC_(a), . . . , CUC_(n)) from the N queues(Q_(a), . . . , Q_(n)) relating to the N destination output modules of amulticast cell (MC), one of the N queues is chosen as a reference queue(Q_(ref)) in which is controlled the service time, referred as thereference service time (SDT_(ref)), of one the N copied unicast cells(CUC_(ref)) corresponding to that multicast cell (MC), and in that, ineach of the N−1 other queues concerned (Q_(c)), the service time(SDT_(c)) of each of the N−1 other copied unicast cells (CUC_(c))corresponding to the same multicast cell (MC) is controlled by selectinga service time instant of the queue concerned (Q_(c)) which is closestto the instant of said reference time (SDT_(ref)) controlled in saidreference queue (Q_(ref)) so that, for each of the N−1 queues concerned(Q_(c)), the temporal distance between the instant of said service timeconcerned (SDT_(c)) and the instant of said reference service time(SDT_(ref)) is at most equal to said service time interval (STI_(c)) ofthat queue concerned.
 2. A method according to claim 1, characterized inthat the reference queue (Q_(ref)) selected is the queue allocated thelowest predetermined cell service bit rate (GOCR_(min)) from the variousservice bit rates (GOCR_(a), . . . , GOCR_(n)) allocated to the N queues(Q_(a), . . . , Q_(n)) relating to the N destination output modules(OTM_(a), . . . , OTM_(n)) of that multicast cell (MC).
 3. A methodaccording to claim 1, characterized in that, for each of the N−1 queuesconcerned (Q_(c)), said service time concerned (SDT_(c)) selected is thefirst service time instant of that queue which follows the instant ofthe reference service time (SDT_(ref)) in said reference queue(Q_(ref)).
 4. A method according to claim 2, characterized in that theselection of said reference queue (Q_(ref)) and the selection of saidservice times concerned (SDT_(c)) in each of the N−1 queues concerned(Q_(c)) are predetermined from the instant of the service time(SDT_(ref′)) that precedes the reference service time (SDT_(ref))retained in the reference queue (Q_(ref)) and in that said service timeconcerned (SDT_(c)) selected for each of the N−1 queues concerned(Q_(c)) is the last service time instant of that queue which precedesthe instant of the reference service time (SDT_(ref)) in said referencequeue (Q_(ref)).
 5. An input module in an asynchronous switching nodecomprising n1 input modules (ITM₁, . . . , ITM_(l), . . . , ITM_(n1))and n2 output modules (OTM₁, . . . , OTM_(j), . . . , OTM_(n2))interconnected by a cell switching network and control means fortransferring each cell from an input module to an output module, aunicast cell (UC) being addressed to a single given destination outputmodule (OTM_(d)) and a multicast cell (MC) being addressed to N givendestination output modules (OTM_(a), . . . , OTM_(n)) of the n2 outputmodules of that node, each input module being provided with means forconverting each multicast cell into N copied unicast cells (CUC_(a), . .. , CUC_(n)) respectively addressed to the N output modules (OTM_(a), .. . , OTM_(n)) and buffer memories for temporarily storing cells in eachinput module, the service time of a normal or copied unicast cell (UC;CUC) in a buffer memory triggering its transfer from the input module tothe destination output module across said switching network, and theservice times of all the unicast cells (UC; CUC) being commanded inqueues (Q₁, . . . , Q_(j), . . . , Q_(n2)) in each input module relatingto each output module (OTM₁, . . . , OTM_(j), . . . , OTM_(n2)), theinput module being controlled so that the respective service times ofthe cells sent from a given queue (Q_(j)) are separated by a timeinterval at least equal to the required service time interval (STI_(j))between two successive cells of the stream of cells supplied by theswitching network to the corresponding output module (OTM_(j)) so that apredetermined cell service bit rate (GOCR_(j)) allocated to thecorresponding queue (Q_(j)) is not exceeded for each stream of cellssupplied to a given output module (OTM_(j)), which input module ischaracterized in that, in order to reduce the time necessary forcommanding the departure of the set of all N copied unicast cells(CUC_(a), . . . , CUC_(n)) from the N queues (Q_(a), . . . , Q_(n))relating to the N destination output modules of a multicast cell (MC),the means in the input module for converting each multicast cell into Ncorresponding copied cells (CUC_(a), . . . , CUC_(n)) include firstcontrol means for choosing one of the N queues as a reference queue(Q_(ref)) in which the departure of one of the N copied unicast cells(CUC_(ref)) corresponding to that multicast cell (MC) is commanded at areference service time (SDT_(ref)) and second control means fordetermining, in each of the N−1 other queues concerned (Q_(c)), thechoice of the service time concerned (SDT_(c)) for commanding thedeparture of each of the N−1 other copied unicast cells (CUC_(c))corresponding to the same multicast cell (MC), by selecting a servicetime instant of that queue concerned (Q_(c)) which is closest to theinstant of said reference service time (SDT_(ref)) controlled in saidreference queue (Q_(ref)) so that, for each of the N−1 queues concerned(Q_(c)), the temporal distance that separates the instant of saidservice time concerned (SDT_(c)) from the instant of said referenceservice time (SDT_(ref)) is at most equal to said service time interval(STI_(c)) of that queue concerned.
 6. An input module according to claim5, characterized in that the first control means are adapted to chooseas the reference queue (Q_(ref)) the reference queue allocated thelowest predetermined cell service bit rate (GOCR_(min)) of the variousservice bit rates (GOCR_(a), . . . , GOCR_(n)) allocated to the N queues(Q_(a), . . . , Q_(n)) relating to the N destination output modules(OTM_(a), . . . , OTM_(n)) of that multicast cell (MC).
 7. An inputmodule according to claim 5, characterized in that the second controlmeans are adapted to select as the service time concerned (SDT_(c)) foreach of the N−1 queues concerned (Q_(c)) the first service time instantof that queue which follows the instant of the reference service time(SDT_(ref)) in said reference queue (Q_(ref)).
 8. An input moduleaccording to claim 6, characterized in that the first control means areadapted to predetermine the reference queue (Q_(ref)) selected from theinstant of the service time (SDT_(ref′)) that precedes the referenceservice time (SDT_(ref)) retained in that reference queue and in thatthe second control means are adapted to preselect as the service timeconcerned (SDT_(c)) for each of the N−1 queues concerned (Q_(c)) thelast service time instant of that queue concerned which precedes theinstant of the reference service time (SDT_(ref)) in said referencequeue (Q_(ref)).